Why are some angles black when I skim my multicam clip on the timeline?

This does sound like a bug. Please report it via Final Cut Pro>Provide Final Cut Pro Feedback...  or this link Feedback - Final Cut Pro - Apple

When you edit your multicam clip into the timeline initially, only one angle is showing. So, you'll only see content where there is media in the same location in the angle editor. As is, if you switch angles you'll see different content in different locations along the multicam clip in the timeline.

What you need to do is to do switching of angles, which creates edit points in the multicam clip in the timeline. You do this by changing angles by clicking an angle in the angle viewer. If you hold the option key and click another angle, the entire portion of the clip under the playhead will switch to the new angle without adding an edit point. You can also switch only video, or both audio and video. There are keyboard shortcuts for doing a lot of this, as well...

In a typical scenario, you edit the multicam clip into a timeline. Then, you start playback and switch (cut to) angles as you go along. You can then go back and change angles on each newly created clip in the multicam.

The user manual is helpful, or you might want to view some videos...

The only time I recall multicam angles going black is when we used GoPro H265 footage. I think all we did was quit and relaunch FCP, but it sounds like you've tried that. If you make an edit to an angle that shows black, does the edit display accurately in the timeline?

Have you tried resetting FCP prefs? We don't use multiple monitors so I don't know if this will mean resetting your monitor layout with FCP.

Whenever FCP crashes or starts acting strangely an easy thing to try is resetting preferences. Hold option and command while launching FCP and choose Delete Preferences. It will open the default Untitled library. Re-open the library you were working on, set the preferences back to the way you like them (disable background rendering, reset Import to Leave Files in Place, etc.) and see if the issue has been resolved.

I believe we understand things. I misspoke earlier. I see in your last screenshot that the top 3 angles in the angle viewer shouldn't be black. I agree with Terry that you might want to save your workspace layout, delete preferences, and try things all on one screen. Other than that, it looks like the top three angles are shifted "out of the frame" (or just missing).

Update:

With my workspace set to use one display, I created a simple, two angle MC clip. I then created a new project with a vertical aspect ratio. I opened the angle viewer and could see both angles.

I then switched to the "Dual Displays" workspace setting and opened the angle viewer again. The top row (both of my angles) showed black but the timeline was correct. I changed the angle layout via the Settings menu to another configuration (9 angles) and then back to 4 angles and the angles then appeared in the angle viewer. Seems like a UI glitch/bug (maybe to do with changing workspaces or that the Dual Display workspace isn't initially configured properly???)...

Did you try things with a one display workspace layout? This definitely seems like a bug. You might want to consider sending feedback to Apple.

As a workaround, you could try editing with a horizontal project, maybe even using an overlay to simulate the vertical framing (this may depend on other settings for your project/timeline).